1. Field of the Invention
This disclosure relates generally to devices for estimating blood alcohol content from a breath sample, and more particularly, to fuel cells for use in estimating blood alcohol content from a breath sample.
2. Description of the Related Art
An alcoholic beverage is a drink containing ethanol, commonly known as alcohol, although in chemistry the definition of alcohol includes many other compounds. Alcohol, specifically ethanol, is a psychoactive drug and is a powerful central nervous system depressant with a range of side effects.
Alcohol has a biphasic effect on the body, which is to say that its effects change over time. In the initial stages of intoxication, alcohol generally produces feelings of relaxation and cheerfulness. Further consumption however affects the brain leading to slurred speech, blurred vision, clumsiness and delayed reflexes, among other coordination problems. This condition is commonly referred to as intoxication or drunkenness, and eventually subsides when the alcohol has fully metabolized in the body.
When a human drinks alcohol, the alcohol housed in the stomach passes into the bloodstream. Cell membranes are highly permeable to alcohol, so once alcohol is in the bloodstream it can diffuse into nearly every biological tissue of the body. Once in the bloodstream, the alcohol circulates to the brain, resulting in intoxication, loss of inhibition and impairment of motor skills such as driving a vehicle. The amount of alcohol consumed and the circumstances surrounding consumption play a large role in determining the extent of an individual's intoxication. Examples of such circumstances include the amount of food in the stomach at the time of alcohol consumption and the hydration level of the individual at the time of consumption, among others.
Due to the coordination impairment and other symptoms associated with intoxication and drunkenness, most countries have laws against drunk driving, i.e., driving with a certain concentration of ethanol in the blood. The legal threshold of blood alcohol content ranges from 0.0% to 0.08%, depending on the jurisdiction. Punishments for operating a vehicle over the legal limit in a given jurisdiction generally include fines, temporary loss of an individual's driving license and imprisonment. Creation of these laws has led to a market for devices to accurately measure the blood alcohol content of individuals operating motor vehicles.
Blood alcohol content (BAC) or blood alcohol concentration is the concentration of alcohol in the blood (weight per unit volume). While blood alcohol content can be directly measured in a hospital laboratory setting, it is more common for it to be measured in law enforcement situations by estimation from an individual's breath alcohol concentration using a breath alcohol testing machine.
In the world of alcohol-breath testing and related fields of alcohol testing, one of the most common configurations uses fuel cells as alcohol sensors, with the assembly allowing breath, air, gas, or vapor to be passed into the fuel cell. In its simplest form, the alcohol fuel cell is a wafer consisting of a chemically inert porous material coated on both sides with thin layers of catalyst such as platinum (forming upper and lower platinum electrode layers.) The porous material sandwiched between the two platinum layers contains a liquid acid-electrolyte. The electrolyte allows charges to move between the two electrode layers of the wafer. Those skilled in the art would understand that the chemically inert porous material, filled with a liquid acid-electrolyte, could be replaced by, under certain circumstances, a solid electrolyte element.
Round, platinum wire electrical connections are then applied to the platinum electrodes and connected to an external circuit. In this regard, the wafer is generally a solid, planar shape allowing for wire to electrode connections at any point on the wafer. The entire fuel cell sensor is mounted in a plastic case, which is provided with a gas inlet that allows a breath sample to be introduced into the assembly. The basic configuration is as described above, and illustrated in FIG. 1.
In the fuel cell sensor, the top platinum electrode (the one closest to the gas inlet) oxidizes any alcohol in the breath sample to produce acetic acid in a 2 step process (ethanol→acetaldehyde→acetic acid) and which also produces free electrons in the process. Hydrogen ions (H+) are also freed in the process, and migrate to the lower platinum electrode of the cell, where they combine with atmospheric oxygen to form water, resulting in a deficiency of electrons on the lower electrode equal to the excess of electrons produced on the upper surface. Because the two electrode surfaces are connected electrically, a current flows through this external circuit to neutralize the charge. With suitable amplification, this current is a precise indicator of the amount of alcohol consumed by the fuel cell, as the number of electrons produced is directly and linearly proportional to the number of alcohol molecules arriving at the catalyst surface. With the number of alcohol molecules, the blood alcohol content can then be determined. This process is illustrated in FIG. 2.
For the fuel cell sensor to operate properly, the wires connecting the electrodes must have a low ohm connection with the platinum electrodes on the wafer. Otherwise, a high resistance connection hinders the operation of the fuel cell and makes it less accurate and slower. The “wires” can take on a variety of forms, but as noted above, round, solid platinum wires are the most common. They are generally considered the best, most readily available, and most versatile. Other forms can be used besides round wire, such as wire mesh or flat ribbon. Additionally, other materials can be used besides platinum, such as gold or plated materials, but resistance to the harsh acidic environment of a fuel cell can limit the number of choices of conductors. In any event, the wire should not disrupt the thin layer of platinum, and should take into consideration the well understood effects of dissimilar metal junctions.
As noted herein, it is common practice by those skilled in the art to use small round platinum wires. platinum, however, is an expensive commodity, so using as little wire as possible can be important economically. The platinum wire only needs to contact a portion of the platinum surface. Thus, at times, the platinum wires are transitioned to some other, more economical conductor once the wires have exited the harsh acidic environment inside the fuel cell case/assembly.
Moreover, there exists a considerable price pressure in the marketplace for alcohol fuel cell sensors, specifically in the breath-testing field. Most manufacturers use low cost and simple components in the construction of such fuel cell sensor assemblies. Added complexity of design can result in higher costs for tooling, parts, and labor. For all these reasons, a common simple construction assembly used by many manufacturers can be generally described as follows and as illustrated in FIG. 3: a first wire (a) is added to a lower plastic case (b); a platinum coated disk (c) is added on top of the first wire (a) and case (b); a second wire (d) is added over the disk (c); and a upper plastic cover (e) is added to seal the assembly.
It is important that the assembly be sealed for a couple of reasons. First, it ensures that no electrolyte can leak out of the assembly. Second, gas can only enter the assembly through the inlet. For sealing, manufacturers use glues, epoxies, ultrasonic welding, flexible seals, and other types of sealing methods known to those skilled in the art. In order for the assembly to be fully sealed, the case halves must be sealed together, the wires must be sealed to the case, and the periphery of the disk must be sealed to the inside of the case half.
Importantly, the wires must have good contact with the platinum surface of the electrodes. This contact is most easily accomplished by a clamping force that “squeezes” the platinum disk between the wires—often the platinum wires can actually compress the platinum electrodes and embed in that surface to some degree. Additionally, this contact and force needs to be maintained throughout the life of the sensor.
As shown in FIG. 3, with many of the prior art assemblies, as the wires are added to the assembly, some of the parts in the assembly become canted. Not only does this make the assembly more difficult, but the wires may lose contact with the platinum electrode surface over time, resulting in non-functionality of the fuel cell.
There have been attempts in the prior art to ensure this contact, but they all have their own problems. As shown in FIG. 4, one option is to add spacers or compensating elements in areas where the wires are not present. This design, however, is much more complicated and requires a critical equivalence between the dimensions of the compensating elements and the platinum wire in order to properly function. Additionally, this design results in difficulties in maintaining the same clamping force across a batch of assemblies, with some of the manufactured assemblies inevitably clamping the platinum wire to the electrode surface better than others. Although the compensating elements could be spring-loaded in some form to improve the design to ensure proper and even clamping force, such spring-loading only complicates the assembly even further.
A second option is to shape the wires into loops (or portions of loops) that encircle the circumference of the wafer or to span the entire length or diameter of the wafer, as shown in FIG. 5. This is a more reliable option (if used with the proper clamping force) in that it provides more assurance that the platinum wire will remain in contact with the electrode surface, as the housing clamps the outside of the wafer and locks the wires into place. However, the circumferential loops and/or lengthened wire require much more platinum wire which adds significant expense to the assembly.
In summary, fuel cell assemblies generally require some type of wire (e.g., platinum wire) to be attached to platinum electrodes in a secure, low resistance manner. One type of assembly uses a permanent clamping force to hold the electrodes and wires in close electrical contact, with the expensive wires generally spanning nearly the entire length, diameter, or circumference of the wafer. The other requirements of the assembly (such as simplicity of parts, sealing, low cost, and ease of assembly), however, can result in a design with significant risk in the reliability of the internal electrical connection. The largest risk is that a less than optimal electrical connection made during the assembly process may only manifest itself later during field use, resulting in a non-functional breath tester.